The mechanisms involved in aging-related osteoporosis remain poorly defined, however, recent studies from our
group, and others, suggest that it is a stem cell disease. We have been studying the role of the cytokine SDF-1
(CXCL12) and Hdac3 in bone marrow mesenchymal stem/stromal cell (BMSC) function and bone homeostasis.
A critical problem is that aging triggers impaired localization, proliferation, survival, and differentiation of the
osteogenic progenitor cell population in the bone marrow (BM), specifically BMSCs. A critical barrier to
preventing these changes is identifying key regulatory pathways, and being able to alter or correct them. We
have identified BMSC epigenetic changes in the microRNA miR-29b-1-5p, that appears to cross-talk with histone
deacetylase 3 (Hdac3), a second epigenetic regulatory system. Further, we have identified that the oxidized
tryptophan (TRP) metabolite kynurenine (KYN) alters these specific epigenetic regulatory molecules in BMSCs,
which in turn directly, and via SDF-1 signaling pathways, affect cell survival, osteogenesis, osteoblastic lipid
storage, and bone formation. Significantly, miR-29b-1-5p, which increases with aging in BMSCs, belongs to the
miR29 family of miRNAs that have been shown to be critical in extracellular matrix homeostasis and
osteogenesis. One novel aspect is that miR29b-1-5p is the “passenger strand”, which is normally thought to be
degraded leaving the complementary miR-29b-1-3p “guide strand” as the functional miRNA; with aging, however,
our data suggest that this passenger strand is an important effector of BMSC dysfunction. Importantly, miR-29b-
1-5p, which targets SDF-1, is upregulated by KYN while Hdac3 is down regulated leading to increased lipid
storage and dysfunction in BM osteoprogenitor cells and OBs. Understanding the upstream mechanisms that
drive these previously unknown age-associated changes in miR-29b-1-5p and Hdac3 is a critical goal because
we have demonstrated that these two interacting systems regulate SDF-1 expression and suppress BMSC
osteogenesis and survival pathways while increasing BM fat storage. We propose to test the hypothesis that
the elevated, age-related levels of KYN drive the increased expression of miR-29b-1-5p and inhibition of Hdac3
expression, with downstream effects on bone homeostasis via SDF-1 and its receptor CXCR4, or directly by
targeting additional osteogenic genes both in BMSCS and OBs. Our objectives are to test new hypotheses
derived from our findings by manipulating the miRNAs and Hdacs we identified as changing with age in
human/murine BMSCs to determine their effects on bone formation and turnover in vivo and at the molecular
level on BMSC osteogenic function. We will test novel methods to reduce expression of these molecules,
including inhibiting KYN generation, inhibiting KYN signaling pathways, or delivering synthetic anti-miRNAs to
inhibit miR29b-1-5p. The impact of this project will be to clarify the roles of age-associated epigenetic changes
in cross-talking BMSC miRNA and Hdac systems with the goal of identifying novel targets for reducing, or
reversing, age-related bone loss and osteoporosis.
